It is known to carry out an all-sided coating of workpiece articles, such as structural shapes, with corrosion-resistant or protective coatings, with lacquers or paints to improve surface quality or with other liquid-coating agents designed to constitute a specialty treatment for the articles in a continuous vacuum coating apparatus having a generally L-shaped housing with a lower horizontal relatively short housing part and an upright relatively long housing part.
In the lower horizontal and relatively short housing part, forming the short leg of the L, a medium supply is provided together with means for displacing, circulating and cleaning the flowable coating media.
The upper end of the upright or long leg of the L is provided with a vacuum generator which applies suction at a lateral opening of the upright portion of the housing which is connected to a vacuum-coating chamber.
An air-guide plate in the upright portion of the housing extends from the location above this opening at an inclination downwardly to ensure that air drawn out of the vacuum-coating chamber is directed downwardly along the guide plate before being permitted to rise in the upright leg of the housing to be discharged by the vacuum generators.
An apparatus of this type is disclosed, for example, in the German patent document DE-OS 37 40 201. This apparatus combines a number of advantages. For example, it can be rapidly set for workpieces of different sizes and shapes. It also permits vacuum-coating application to fields in which earlier vacuum-coating apparatus was not applicable.
The separation of particulates and residues from the evacuated air also was especially advantageous in this system by comparison with earlier arrangements wherein, for example, the vacuum-application chamber was provided directly above the liquid-supply vessel and seated thereon and wherein the vacuum generator was connected to one end of the latter vessel In these arrangements, the evacuated air stream from the vacuum-coating chamber, rich in finely dispersed atomized excess liquid was fed back to the supply vessel. However, since the evacuated air stream also passed along the free upper surface of the liquid in the vessel, liquid droplets could be entrained and hence the loading of the evacuated air stream with liquid could be increased.
The excess loading of the evacuated air stream with liquid substantially increased the cost of air cleaning as well as the losses of the liquid from the system.
In the improved arrangement with its L shape, however, the vacuum-coating chamber is disposed laterally on the upright leg of the housing. The vacuum generators provided at the upper end of this housing part are located very far from the supply of liquid, so that an intensive suction-air stream is not generated above the liquid surface.
The air-guide plate extending over the connecting opening between the upright leg of the housing and the vacuum-coating chamber does indeed deflect the suction-air stream downwardly before it can pass upwardly in the upright portion of the housing, but in the area in which this deflection takes place, there is little tendency to entrain liquid from the supply and any contact between the deflected air and the liquid is minimal.
The deflection does cause the air to rise opposite the inertia of the droplet particles and the gravitational force applied thereto so that the evacuated air is, to a large measure, cleaned of liquid before and as the air passes upwardly through the upright housing part.
Nevertheless, it is found in such systems that the air as it approaches the vacuum generators remains wet with the liquid components.
Directly upstream of the vacuum-generator units, moreover, it is customary to provide a filter for trapping any droplets before the air reaches the vacuum pump or pumps. In the earlier system, this filter is rapidly wetted with the liquid and thus operates as a wet filter. As a consequence, the discharged air or waste air practically always contains residues of the liquid.
This fact can be associated with needless expense since the residues in the waste air can withdraw substantial amounts of liquid from the liquid circulation path.
In apparatus of this kind, moreover, the short leg of the housing can be formed with an inclined bottom for the liquid supply. This inclined body can have at its lowest point a recess or clapper bottom at which the liquid-coating material is withdrawn by a feed pump and which is covered by a fine sieve at its top. A partial stream of controllable strength is branched from the liquid circulation path and is used to flush the fine screen from solid residues and to maintain flow through this screen.
It will be appreciated that workpieces can never be entirely free from dust and machining residues and that the liquid and especially excess quantities thereof, often entrain microfine solid particles with them. This can give rise to increasing operation times and increasing consumption of the liquid over which there may be a significant rise in the presence of such solid particles. This type of enrichment can plug narrow nozzles and detrimentally affect the surface qualities which are obtained, especially in lacquer applications.
It has been proposed to include cartridge filters in the liquid circulation path. Such cartridge filters have filtering units which are traversed inwardly by the liquid, i.e. from the exterior to the interior Replacement of the filter units and cleaning of the filter vessel from solids must be carried out relatively frequently and often is a problem.
There is little advantage to use filter units which have a more coarse porosity to increase the intervals between change because, in that case, the finest solid particles tend to be circulated in the system and may reach the workpiece surfaces to be detrimental to the coating or may provide an increased load in the waste air which must be removed by waste-air cleaning operations.